1. Field of the Invention
The invention is related to MODFET structures and, in particular, to a MODFET structure having an etch stop layer to limit the thickness of the doped semiconductor layer.
2. Background
In conventional modulation doped field effect transistors, MODFET's, the threshold voltage V.sub.th depends upon the thickness and donor concentration of the doped semiconductor layer. For example, the threshold voltage for a doped aluminum gallium arsenide (AlGaAs) layer having a doping concentration of 1.0.times.10.sup.18 varies as much as 6.0 millivolts per angstrom of thickness. Because of this, it is difficult to control the threshold voltage. Recently, as taught by T. Wada et al, Jap. J. Applied Physics, Vol. 26, pp. 213, 1985, semiconducto-insulator-semiconductor (SIS) devices on undoped aluminum gallium arsenide on an undoped gallium arsenide buffer layer have been fabricated which overcome this problem. In addition to good threshold voltage control, these devices show no I-V collapse which is observed in conventional structures. The MODFET structure disclosed herein is a modification of the conventional MODFET having the advantages of the SIS structure.